18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS SURFACE FUNCTIONALIZATION OF MULTI-WALLED CARBON NANOTUBES WITH RANDOM COPOLYMER: SYNTHESIS AND CHARACTERIZATION Lingappan Niranjanmurthi 1 , Sung Yong Seo 2 , Kwon Taek Lim 1 * Division of Image System Science and Engineering, Pukyong National University, Busan 608-739, Korea Department of Chemistry, Pukyong National University, Busan 608-739, Korea (ktlim@pknu.ac.kr) Keywords : Multi-walled carbon nanotube, Random copolymer, Polymer nanocomposites, P oly (3-thiophene ethanol-co-hexylthiophene), Abstract aggregation of CNTs in solutions or polymer The copolymer poly (thiophene ethanol-co-hexyl matrices and fully utilizing their unique properties. thiophene) (P3ET-co-P3HT) and its composites with Various approaches have been developed for multi-walled carbon nanotubes (MWNTs) were enhancing dispersion, including covalent sidewall prepared via in-situ oxidative polymerization. The functionalization of CNTs [6], noncovalent sidewall functionalization of MWNTs was achieved by functionalization of CNTs [7] in situ polymerization introducing active sites (-COOH) onto the surface of of monomers with nanotubes [8] polymer wrapping MWNTs by oxidizing them with the mixture of [9] and solution blending [10]. In particular, surface H 2 SO 4 and HNO 3 (1:3). The oxidized MWNTs were functionalization of CNTs either covalent or non- reacted subsequently with SOCl 2 to get acylated covalent functionalization is one of the major MWNTs. The acylated MWNTs were grafted with techniques developed to improve the CNT 3- thiophene ethanol via “ester linkage” followed by dispersion or solubilization. It involves the physical copolymerization with 3-hexyl thiophene using adsorption of polymers or surfactants on the anhydrous FeCl 3 as an oxidizing agent. The structure nanotube surface [11]. and morphology of the nanocomposites were In recent years, polymeric materials and their characterized by FTIR, XPS, TGA, HR-TEM and composites with CNTs have found extensive FE-SEM. The doping function of MWNTs in the applications in organic electronics [12,13]. The composites was analyzed using UV-vis and PL formation of CNTs/polymer composites has been spectroscopy. Morphological observations revealed explored for possible improvement in the electrical that the functionalized MWNTs were uniformly and mechanical properties of polymers. In particular, dispersed in the copolymer matrix. UV-Vis spectra composite materials based on the coupling of showed that the absorption peak of the copolymer conducting polymers and CNTs have been shown to was red-shifted by 20nm. The emission spectra possess properties of the individual components with indicated that the PL intensity of the nanocomposites a synergistic effect. CNTs can be used as ideal was much higher than that of the copolymer. reinforcing agents for high performance polymer composites. Numerous molecular electronic devices, 1 Introduction including light-emitting diodes [14] photovoltaic Carbon nanotubes (CNTs) have various applications cells [15] transistors [16] sensors [17] and memories in many fields of nanotechnology due to their unique [18] have been demonstrated. mechanical, thermal, and electrical properties [1]. In this article, we report a new strategy for the This wide range of properties makes CNTs synthesis of nanocomposites consisting of multi- potentially attractive tools for a variety of walled carbon nanotubes (MWNTs) with a random applications, from nanoelectronics to biomedical copolymer poly (3-thiophene ethanol-co- devices [2 – 4]. The effective utilization of CNTs in hexylthiophene) (P3ET-co-P3HT). The MWNTs-g- nanocomposite applications depends strongly on the (P3ET-co-P3HT) composites were prepared via in- ability to disperse CNTs homogeneously throughout situ oxidative polymerization in the presence of a matrix without destroying the integrity of CNTs. functionalized MWNTs. The structure and However, the practical applications of CNTs are morphology of the resulting MWNTs- g -(P3ET-co- impeded by their tendency to agglomerate and their P3HT) composites were investigated in detail using insolubilities in organic solvents and water [5]. FTIR, XPS, HRTEM, XRD, FT-IR, UV – vis and PL Surface functionalization of CNTs has been actively spectroscopy. pursued, which is crucial for preventing the
2 Experimental details The reaction mixture was stirred continuosly under a nitrogen atmosphere at room temperature for 24 h. 2.1 Materials After polymerization, the reaction mixture was MWNTs (ca. 90% purity) with a diameter range of poured into 200 mL methanol in order to remove the 10-20 nm were obtained from Iljin Nanotech Co. Ltd ungrafted polymers and FeCl 3. The precipitate was (Korea). 3ET and 3HT were purchased from Sigma washed several times with acetone to minimize the Aldrich (Korea) and used as received. Anhydrous FeCl 3 content in the composites and the composites CHCl 3 and methanol were obtained from Junsei were dried in a vacuum oven at 50°C. The (Japan) and distilled before use. copolymer was synthesized with the same procedure for the comparison. 2.2 Purification of MWNTs 2.6 Characterization Pristine MWNTs were purified by thermal and acid FT-IR analysis of the MWNT – COOH and MWNTs- treatment. The pristine MWNTs were heated in air at 650°C for 2h, and then cooled and refluxed with g-(P3ET-co-P3HT) composites was done using conc. HCl for 24h. After the mixture was diluted Perkin-Elmer GX (USA) in the frequency range of 4000 – 400 cm − 1 . The FTIR measurements of the with de-ionized water, the precipitate was collected and washed repeatedly with distilled water. The powder samples were performed in the form of KBr product was filtered and dried in a vacuum oven for pellets. XRD measurements were performed in the 2θ region on a Rigaku diffractometer using nickel- 12 h at 40°C. filtered Cu Kα radiation . The morphologies of the 2.3 Surface functionalization of MWNTs nanocomposites were examined by using a 1.0 g of purified MWNTs were suspended in 150 transmission electron microscopy (TEM, JEOL mL of the mixture of conc. H 2 SO 4 and HNO 3 (1:3 JEM-2000EX) and a field emission scanning v/v), and the dispersion was sonicated at room electron microscopy (FESEM) equipped with in situ temperature for 8 h. The mixture was diluted with energy dispersive X-ray (EDX) spectra (Hitachi, S- 300 mL of de-ionized water and filtered through a 2700 model microscope, Japan). Samples for TEM 0.1mM polytetrafluoroethylene filter membrane. were deposited onto carbon-coated copper electron The filtered product (MWNTs – COOH) was washed microscope grids and dried in air. Thermal study of with distilled water until the pH value of the filtrate the composites was studied using Thermo became neutral. gravimetric analysis (Perkin Elmer) Pyris1 at a heating rate of 10°C/min under nitrogen atmosphere. 2.4 Acylation of MWNTs The chemical structure of the composites was MWNTs – COOH (0.6 mg), was reacted with 100 mL carried out on a ESCA 2000 XPS (Thermo VG of SOCl 2 at 70 ◦ C for 24 h under reflux to convert Scientific) using a monochromatic (Mg KR = 1253.6 the surface-bound – COOH groups into acyl chloride eV) source. UV – visible spectra were obtained using groups. Any residual SOCl 2 was removed by rotary a Perkin Elmer Lambda 40 ultraviolet – visible (UV – evaporation, and the solids that were subsequently Vis) spectrometer. PL spectra were recorded on a F- obtained were filtered and washed with anhydrous 4500 spectrofluorometer (Hitachi, Japan). THF. Lastly, the filtrate was dried under vacuum at 3 Results and discussion room temperature for 4 h to give acyl chloride functionalized MWNTs (MWNTs – COCl). FT-IR spectra were used to characterize the functional groups of polymers and MWNTs after 2.5 Preparation of MWNTs – g – (P3ET – co – P3HT) modification. Fig.1 shows the FTIR spectra of the Scheme 1 shows the chemical reactions used for the oxidized MWNTs, pure copolymer and composites. preparation of the MWNTs – g – (P3ET – co – P3HT) The functionalized MWNTs (Fig.1a) showed a strong peak at 3400 cm − 1 due to the OH stretching composites. In a typical experiment, 0.5 g of MWNTs – COCl was added into 50 mL of CHCl 3 and mode of the COOH group. A sharp peak at 1740 cm − 1 was assigned to the C=O stretching sonicated for 30 min. 0.25 g of 3-ET was added dropwise to the mixture. 0.25 g of 3-HT and 2 g of vibration, suggesting the formation of COOH groups anhydrous FeCl 3 were added into the above mixture. on MWNTs. The characteristic peaks at 1550 and
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